Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography.
Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made. A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of in order, a backing or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often, a protective cover sheet. After processing, the resulting surface of the printing plate has three-dimensional relief pattern, which typically comprises a plurality of printing dots that reproduce the image to be printed.
The support (or backing) layer lends support to the plate. The support layer can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. One widely used support layer is a flexible film of polyethylene terephthalate.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. As used herein, the term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional relief pattern of cured material. Exemplary photocurable materials include those disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may also be used.
Photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, “actinic radiation” is radiation that is capable of polymerizing, crosslinking or curing the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light, and the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate.
In a “digital” or “direct to plate” process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is generally a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are then ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
Processes for forming relief image printing elements typically include the following processing steps:                1) Image generation, which may be mask ablation for digital “computer to plate” printing plates or negative production for conventional analog plates;        2) Back exposure to create a floor layer in the photocurable layer and establish the depth of relief;        3) Face exposure through the mask (or negative) to selectively crosslink and cure portions of the photocurable layer not covered by the mask, thereby creating the relief image;        4) Development to remove unexposed photopolymer by solvent (including water) or thermal development; and        5) If necessary, post exposure and detackification.        
Removable coversheets are also typically provided to protect the photocurable printing element from damage during transport and handling. Prior to processing the printing elements, the coversheet(s) are removed and the photosensitive surface is exposed to actinic radiation in an imagewise fashion. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer solvent (or thermal development) removes the unexposed areas of the photopolymerizable layer, leaving a three-dimensional printing relief that can be used for flexographic printing.
As used herein “back exposure” refers to a blanket exposure to actinic radiation of the photopolymerizable layer on the side opposite that which does, or ultimately will, bear the relief. This step is typically accomplished through a transparent support layer and is used to create a shallow layer of photocured material, i.e., the “floor,” on the support side of the photocurable layer. The purpose of the floor is generally to sensitize the photocurable layer and to establish the depth of relief.
Following the brief back exposure step (i.e., brief as compared to the imagewise exposure step which follows), an imagewise exposure is performed utilizing a digitally-imaged mask or a photographic negative mask, which is in contact with the photocurable layer and through which actinic radiation is directed.
The type of radiation used is dependent on the type of photoinitiator in the photopolymerizable layer. The digitally-imaged mask or photographic negative prevents the material beneath from being exposed to the actinic radiation and hence those areas covered by the mask do not polymerize, while the areas not covered by the mask are exposed to actinic radiation and polymerize. Any conventional sources of actinic radiation can be used for this exposure step. Examples of suitable visible and UV sources include carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flash units, electron beam units, photographic flood lamps and LEDs.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or thermal development, which typically uses heat plus a blotting material. The resulting surface has a three-dimensional relief pattern, which typically comprises a plurality of dots that reproduces the image to be printed. After the relief image is developed, the resulting relief image printing element may be mounted on a press and printing commenced.
The shape of the dots and the depth of the relief, among other factors, affect the quality of the printed image. In addition, it is very difficult to print small graphic elements such as fine dots, lines and even text using flexographic printing plates while maintaining open reverse text and shadows. In the lightest areas of the image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represent the density of the image. In both cases, it is necessary to print very small dot sizes to adequately represent the highlight areas. In addition, it may also be desirable in some instances to also include very small dots on the printing element that do not print but merely support the relief image.
Maintaining small dots on flexographic plates can be very difficult due to the nature of the platemaking process. In digital platemaking processes that use a UV-opaque mask layer, the combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (i.e., the dot is not “held” on plate and/or on press). Alternatively, if the dots survive processing they are susceptible to damage on press. For example small dots often fold over and/or partially break off during printing, causing either excess ink or no ink to be transferred.
U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub. No. 2011/0079158 to Recchia et al., the subject matter of each of which is herein incorporated by reference in its entirety, describe a particular set of geometric characteristics that define a flexo dot shape that was found to yield superior printing performance. These geometric characteristics include, but not limited to (1) planarity of the dot surface; (2) shoulder angle of the dot; (3) depth of relief between the dots; and (4) sharpness of the edge at the point where the dot top transitions to the dot shoulder.
Upon UV-curing, the surface properties of a given photopolymer can be quite different from its bulk properties. This is mainly because oxygen inhibition strongly affects the surface layer upon UV exposure, thus disproportionately suppressing the curing reaction in the surface layer compared to the bulk. As a result, the most desirable properties for end-use product performance are not attained. In addition, poor surface cure can significantly alter the size and shape of the relief features formed in the photopolymer by UV curing.
As described for example in U.S. Pat. Pub. No. 2014/0004466 to Vest et al., the subject matter of which is herein incorporated by reference in its entirety, a process has also been developed for digital plating making in which an oxygen barrier membrane is laminated to the top of the one or more photopolymer layers and is imaged through the oxygen barrier membrane to create printing dots having a flat top and sharp dot edges.
Flat tops allow much higher ink transfer volume in the print compared to round tops, especially for solid areas and line work that have been screened with a high frequency pattern. However, for some applications, flat top dots have some disadvantages. For example, screen dots printed with flat tops for low percentage screen dot areas in highlight areas of a print may end up with too much ink transfer.
Thus, it would be desirable to provide an improved method of producing a relief image printing element having both flat top dots and round top dots on the same printing element or vignette in a digital printing plate process without the need for additional processing steps and without the need for specialized equipment or procedures.